
How does Epidemiology define a
comparative cohort study?
…it depends on what Epidemiology textbook you read…
“In a retrospective cohort study…the investigator identified the cohort of
individuals based on their characteristics in the past and then reconstructs
their subsequent disease experience up to some defined point in the most
recent past or up to the present time”
--Kelsey et al, Methods in Observational Epidemiology, 1996
“In a cohort study, a group of people (a cohort) is assembled, none of whom
has experienced the outcome of interest, but all of whom could experience
it…On entry to the study, people in the cohort are classified according to those
characteristics (possible risk factors) that might be related to outcome. These
people are then observed over time to see which of them experience the
outcome.”
--Fletcher, Fletcher and Wagner, Clincal Epidemiology – The
Essentials, 1996
“In the cohort study’s most representative format, a defined population is
identified. Its subjects are classified according to exposure status, and the
incidence of the disease (or any other health outcome of interest) is
ascertained and compared across exposure categories.”
--Szklo and Nieto, Epidemiology: Beyond the Basics, 2007
“In the paradigmatic cohort study, the investigator defines two or more groups
of people that are free of disease and that differ according to the extent of
their exposure to a potential cause of disease. These groups are referred to as
the study cohorts. When two groups are studies, one is usually though of as
the exposed or index cohort – those individuals who have experienced the
putative causal event or condition – and the other is then thought of as the
unexposed or reference cohort.”
--Rothman, Modern Epidemiology, 2008
“Cohort studies are studies that identify subsets of a defined population and
follow them over time, looking for differences in their outcome. Cohort
studies generally compare exposed patients to unexposed patients, although
they can also be used to compare one exposure to another.”
--Strom, Pharmacoepidemiology, 2005